This invention relates to aircraft landing systems and more particularly to a method and apparatus for determining whether a hard landing has occurred, assisting in determining whether further inspection for structural damage is necessary.
The current process for deciding that a xe2x80x9chard landingxe2x80x9d has occurred is based on a subjective assessment by the flight crew. Because of the lack of reliable quantitative data, errors are made in this assessment. As a result, an airplane may be grounded unnecessarily, at a considerable cost of time and money, or conversely, a damaged airplane can continue in service. This invention will provide quantitative data that will assist in determining if a structural inspection is required.
The technical literature regarding methods for determining hard landings can be divided into two classes. The first, and most often cited method is to utilize kinetic measurements (acceleration, velocity or displacement indications). The second method is to utilize force measurements (pressure or stress/strain indications). Only the second method of measurement addresses the physical phenomena causing the structure to fail.
Attempts to utilize kinetic measurements (airplane vertical deceleration or sink rate, for example) have failed because the kinetic conditions are only ancillary to parameters and not the cause of the structural damage. The dynamics that occur in a hard landing are very complex. Forces are applied to the airplane structure from a multitude of causes. Examples of these causes include arresting the vertical decent of the airplane, wing lift at touchdown less than the weight of the airplane, momentum of the airplane about its roll axis, spin up of the wheels and tires with the associated spring back and side forces due to airplane yaw at touchdown. An accelerometer or other kinetic instrument will not measure the effect of many of these forces. Recent accident investigations have shown that flight data recorder information did not show that a hard landing occurred, (using the conventional ten feet per second sink rate criteria) even though the airplane impacted the ground with a force that caused structural failure.
An object of this invention is to avoid the problems the kinetic measurements have by measuring the stress/strain occurring in critical elements of the structure. These measurements are independent of the cause of the forces, they indicate actual stress/strain being applied to the structure. It is the stress/strain in the structure that can cause it to suffer damage.